In one type of nuclear reactor, control rods are selectively inserted and withdrawn from a nuclear reactor pressure vessel for controlling the operation thereof. Each of the control rods is typically positioned by a conventional control rod drive which includes a ball screw or spindle threadingly engaging a ball nut for raising and lowering the ball nut as the spindle is rotated either clockwise or counterclockwise. A hollow piston rests upon the ball nut at one end thereof and at its other end is conventionally joined to the control rod. Displacement of the ball nut provides displacement of the piston which in turn inserts or withdraws the control rod in the reactor vessel.
In order to achieve faster insertion of the control rod than could be obtained by normal rotation of the ball spindle, which is conventionally referred to as a scram operation, a rapid flow of high-pressure water is injected through the control rod drive past the piston for lifting the piston off the ball nut in a relatively short time for quickly inserting the control rod into the reactor vessel. The high-pressure water is channeled to the control rod drive through a scram line pipe attached to a high-pressure water accumulator.
In one type of occurrence which allows for rapid backflow of the water past the piston, due to, for example, a break in the scram line, the backflow may cause a large reverse pressure on the piston which in turn provides a back force on the control rod ball nut. This back force can cause reverse rotation of the ball spindle with corresponding withdrawal of the control rod. Withdrawal of one of the control rods due to such a backflow occurrence may cause damage to adjacent fuel in the reactor vessel, requiring replacement thereof leading to undesirable down time of the reactor and economic losses.
In order to prevent the above occurrence, a conventional electromechanical brake is provided in the control rod drive for holding the ball spindle from rotating unless the brake is energized. The brake is sized for restraining rotation of the ball spindle against such forces due to backflow of water over the piston when the control rod drive motor is not operating. And, when the control rod drive motor is operating, the motor itself is sized for providing adequate torque for resisting the forces due to the backflow of water in the event of the above-described occurrence.
The scram-line water provided to the control rod drive is conventionally discharged from the housing of the drive into the main reactor pressure vessel. Under normal operating conditions the scram-line water flows continuously as purge flow at a pressure greater than that of the reactor coolant flowing inside the reactor vessel. In a scram operation, the accumulator discharges and thus increases the pressure of the scram-line water. The reactor coolant normally flowing inside the reactor vessel in an exemplary boiling water reactor has a pressure of about 73 kg/cm.sup.2 (1,040 psi) and a temperature of about 270.degree. C. (518.degree. F.), with the purge flow pressure being correspondingly higher. Accordingly, the control rod drive housing which is joined to the main reactor vessel forms part of the pressure vessel which is subject to high pressure and temperature water. And, during the scram operation, the pressure of the scram-line water channeled into the control rod drive housing has yet even higher pressure.
In an exemplary control rod drive, the ball spindle is rotated by a conventional motor mounted outside of the pressure vessel. Therefore, the ball spindle extends through the pressure vessel and thus requires a suitable seal, such as a conventional packing-type seal assembly for preventing unacceptable leakage of the high pressure, high temperature reactor coolant and purge flow from the control rod housing to outside of the pressure vessel. Such a seal assembly results in a more complex control rod drive, is typically renewed periodically which adds to downtime of the reactor and costs, and may also be subject to occasional undesirable sticking of the packing seal and the ball spindle which temporarily resists rotation of the ball spindle.